1. Field of the Invention
The invention relates to systems and methods for controlling aerodynamic properties of surfaces in general and particularly to systems and methods that employ surfaces that can change their configuration in dimensions of millimeters or less.
2. Description of Related Art
For the Reynolds numbers typical of MAV flight (Rechord=104-105 based on a velocity of order 10 m/s and a maximum linear dimension of 0.15 m), stimulation of transition of the airfoil boundary layer using surface roughness elements can be used to improve vehicle performance by delaying the onset of separation to stronger adverse pressure gradients (i.e. higher angles-of-attack or further downstream locations), thus increasing the peak lift-to-drag ratio at a given flow condition (e.g. FIG. 1 from Gad-el-Hak, “Micro-Air-Vehicles: Can They Be Controlled Better?” AIAA J., Vol. 28, No. 9, (1990) 1537-1552). Conversely, in flow regimes with less stringent pressure recovery requirements or higher Reynolds numbers, the turbulent boundary layers on rough airfoils lead to a reduction in performance compared to their smooth counterparts because of the increased skin friction drag associated with a thickening turbulent boundary layer and the associated increase in susceptibility to turbulent separation.
These arguments suggest that the agility of an MAV would be enhanced if the airfoil design process could incorporate a variably rough surface: one that is designed to be smooth at high Reynolds numbers, but rough when beneficial at lower Reynolds numbers (but not so rough as to unnecessarily thicken the boundary layer). A material that is capable of morphing on the scale of the surface roughness while forming an integral, possibly structural, part of the airfoil skin holds the promise of overcoming both the weight and power restrictions that impede implementation of many current active control schemes and the reduced high Reynolds number performance of permanently rough airfoils.
Advances in smart materials mean that there are an increasing number of modern materials that may be translated to new applications in the fluid mechanics arena. In this collaborative research, we seek to couple a known material instability with knowledge of the surface roughness required to cause boundary layer transition in canonical and applied flows.
Previous work falls into the categories of roughness-induced transition, devices to induce roughness in canonical and applied flow configurations and thin-film/substrate buckling.
The classical studies of the effects of distributed roughness on transition can be used to guide the required roughness amplitudes. For example, the prior art indicates that transition can be induced in a Blasius boundary layer at Reynolds numbers as low as Reδ*˜300 for single three-dimensional roughness elements or Reδ*˜1000 for two-dimensional roughness (transition occurs close to the roughness if the amplitude, k, is of the same order as the local boundary layer displacement thickness, k/δ*˜1, compared to a natural transition Reynolds number for a smooth wall in his experiments of Reδ*˜2600 (where δ* is the displacement thickness at the transition location). The prior art has described applications of MEMs technologies to control of separation on MAV airfoils, indicating that a Gaussian bump with (time-dependent) amplitude of order ten viscous units would provide sufficient local acceleration to suppress incipient separation. More peripherally relevant are previous studies of membrane wing aerodynamics (e.g. Lian 2003) and the computational study of a “dynamic”, or time-dependent, roughness for the control of laminar boundary layer separation from an airfoil (e.g., Honsaker 2005).
There is a need for systems and methods that permit the control of surfaces for use in aerodynamic applications.